How do I change to be annually charged for iCloud storage

There is no annual payment option for iCloud Storage, only monthly. You could add credit to your Apple ID sufficient to cover a year's subscription, though any movies or music purchased would also be drawn from this, so you would have to take that into account. You could buy sufficient iTunes Store (not Apple Store) gift cards and redeem them in iTunes: or you could use your registered payment method to add credit to your account - please see  Add money to your Apple ID balance - Apple Support